CVE-2020-5403: Reactor Netty HttpServer, versions 0.9.3 and 0.9.4, is exposed to a URISyntaxException that causes the connect

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HIGH 7.5REMOTEDOWNGRADE

CVE-2020-5403 is a high-severity flaw (CVSS 7.5) affecting reactor netty. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Reactor Netty HttpServer, versions 0.9.3 and 0.9.4, is exposed to a URISyntaxException that causes the connection to be closed prematurely instead of producing a 400 response.

Why this priority

High severity
Remotely exploitable, no login, no user action

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.2% (top 33%)
CISA exploited list	No
Weakness	CWE-20, CWE-755
Published	2020-03-03
Last modified	2026-09-04

Publisher (CNA): Pivotal Software, Inc. · Vendor: broadcom

Last updated 2026-09-04 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2020-03-03 19:15 UTCPublished CVE record published
2026-09-04 18:59 UTCUpdated Record last modified

broadcom reactor netty DOWNGRADE

Affected: 0.9.3 · 0.9.4

Fixed in: no fixed release known

Safe to downgrade to: a release before 0.9.3

1. Check installed version

apt-cache policy reactor-netty || rpm -q reactor-netty || reactor-netty --version

2. No fixed release: DOWNGRADE to a release before 0.9.3 (Debian / Ubuntu)

apt-cache madison reactor-netty sudo apt install reactor-netty=<VERSION_BEFORE_0.9.3> sudo apt-mark hold reactor-netty

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list reactor-netty sudo dnf downgrade reactor-netty-<VERSION_BEFORE_0.9.3> sudo dnf versionlock add reactor-netty

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2020-5403?

reactor netty: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2020-5403 being exploited?

CVE-2020-5403 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2020-5403 affect?

CVE-2020-5403 affects reactor netty. Severity is high (CVSS 7.5). Reactor Netty HttpServer, versions 0.9.3 and 0.9.4, is exposed to a URISyntaxException that causes the connection to be closed prematurely instead of producing a 400 response.

How urgent is CVE-2020-5403?

Priority 48/100. High severity; Remotely exploitable, no login, no user action

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References

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